1. Field of the Invention
The present invention relates generally to a vibration damper including viscously-damping fluid therewithin and disposed between a vibrating member and a vibrated member in order to damp vibration energy, and more specifically to passageways, formed in a partition which divides the fluid chamber into a larger region and a small region, through which the damping fluid passes when relatively low frequency vibrations are transmitted to the damper.
2. Description of the Prior Art
The background of the present invention will be explained with respect to its application to an engine mounting device used for an automotive vehicle.
Recently, vibration dampers of viscous-damping type have often been used as engine mounting devices for automotive vehicles. In this case, the engine mounting device is disposed between a power unit such as an engine and a vehicle body in order to prevent relatively high-frequency vibrations generated by the engine from being transmitted to the vehicle body and simultaneously to prevent relatively low-frequency vibrations generated by the vehicle body from being transmitted to the engine. The low-frequency vibrations (10 to 20 Hz) are usually generated as a vehicle travels along a road, that is, while the wheels of the vehicle vibrate on road surfaces. In the vibration damper of this type, it is very important to effectively damp the relatively low frequency vibrations generated from the vehicle body, because, if not effectively damped, engine vibrations may be reinforced by the low-frequency vehicle body vibrations.
In the vibration damper described above, the high-frequency vibrations generated when pistons move up and down are usually damped by the elastic material housing forming the vibration damper body, while the low-frequency vibrations generated when the vehicle travels along a road are usually damped by a damping force generated as the viscous fluid flows through a passageway formed in the partition.
In order to effectively damp the high-frequency vibrations, the softest-possible material should be used for the elastic member so that its resonant frequency will be well out of the range of the high frequency vibrations generated by the engine. On the other hand, in order to effectively damp the low-frequency vibrations, the loss factor of the low-frequency vibration system should be increased; that is, the damping fluid should pass through the passageway as easily as possible. Therefore, it is possible to consider that the loss factor of the low-frequency vibration system is roughly proportional to the flow rate of the damping fluid through the passageway.
Although it may be difficult to simply consider the reciprocation of the damping fluid flowing through the passageway as a low-frequency linear vibration system, if assumed so, the diameter of the passageway should be so determined that the resonant frequency of the low-frequency vibration system matches the low-frequency vibrations generated by the vehicle body. On the other hand, since the damping coefficient of the passageway changes according to the diameter of the passageway, it is usually difficult to adapt the diameter of the passageway for an appropriate resonant frequency and with a relatively large damping coefficient. In more detail, if the passagway diameter is increased in order to increase the damping coefficient, the resulting resonant frequency is also increased so that it will no longer match the low frequency vibrations generated by the vehicle body.
Furthermore, if the fluid does not reciprocate through the passageway smoothly or at a high low rate, since the elastic member is made of soft material, the damper itself will deform and thus the damping function will not be effectively utilized.
The shortcomings of a prior-art vibration damper will be described in more detail hereinafter with reference to the attached drawings under Detailed Description of the Preferred Embodiments.